Superabsorbent polymers, also referred to as aqueous fluid absorbent polymers or “SAP”, are primarily used in personal care products, e.g., baby diapers, adult incontinence products and feminine hygiene products. In such applications, superabsorbent polymer particles are incorporated into absorbent structures that contain synthetic and/or natural fiber or paper based woven and nonwoven structures, or toughened masses of fibers, such as fluff pads. The materials used in such structures can quickly absorb aqueous fluids and distribute them throughout the whole absorbent structure. Such structures, in the absence of superabsorbent polymers, have limited absorption capacity, are bulky due to the large amount of material needed to provide acceptable absorption capacity, and do not retain fluid well under pressure. The absorbency and fluid retention characteristics of such absorbent structures are improved by incorporating into them SAP particles that imbibe aqueous fluids to form a swollen hydrogel material.
Manufacturers of personal hygiene articles have long desired to have an absorbent core structure with a minimal thickness yet exhibiting superior absorbency capacity in the absorbent article. A thin absorbent structure is not only more economically favorable due to the reduced unit volume for transport, but also is an important factor for avoiding personal embarrassment when worn. In modern diapers, for example, manufacturers have been increasing the amount of SAP relative to the weight of the cellulose fiber component, and the quantity of the superabsorbent polymer in the diaper or other hygiene article can be 50 weight percent or higher. Various methods have been employed previously to increase the absorption capacity at reduced thickness of such an absorbent core structure. Typically, this is accomplished by using higher amounts of the SAP, but there is then a countervailing loss of wicking capability due to increased gel blocking as well as the reduced amounts of fluff capable of incorporation in such SAP-containing articles. Accordingly, previous approaches are not entirely satisfactory in achieving the desired results.
We have discovered that a microcavity-structured, open-cell foam that is suitably prepared from a frothed, aqueous dispersion of thermoplastic, preferably olefin, polymer particles, and optionally a multiplicity of fibers, exhibits a significantly improved softness, surface appearance and permeability. A microcavity structure allows particles to be placed and contained, in high concentration, in a thin foam composite. We have also found, surprisingly, that the microcavity structure of the present invention not only allows better containment of SAP particles up to high concentrations, but it also helps avoid “gel blocking”. Furthermore, we also have found that treating SAP particles' surfaces with other fine particles, particularly nano-particles, surprisingly helps avoid gel blocking. One embodiment of a microcavity in a foam of the present invention is suitably of a generally cylindrical shape.
A composite of the present invention is technically relatively easy to produce. In addition to the good aqueous fluid absorption capability, its thinness and excellent softness, mechanical stability and flexibility make a composite of the invention particularly useful in diapers and various absorbent hygiene applications.
Manufacturers of personal hygiene articles also desire to have an absorbent article that can be recycled easily. Currently, commercial diapers and adult incontinence articles incorporate the absorbent core structure in such a way that the polymeric compositions and cellulose fiber must be separated first in order to be recycled after cleaning of soilage from them. The recycling process thus has been complicated, and rendered mechanically difficult, as well as uneconomical. The polymeric absorbent core structure of the present invention is considered more readily recyclable because its structure can be easily separated from soilage as well as the polymer components from one another. A diaper of the present invention is not only more economical, but also environmentally is more friendly than previous commercial diapers and other hygiene articles.
Published PCT Patent Application No. WO2005/021622 A2, 10 Mar. 2005 relates to aqueous-based froth compositions comprising dispersed olefin polymer particles, methods for preparation of such froths and the production from them of open-celled foams, laminates, and finished articles.
Foams containing SAP particles have been prepared from, for example, carboxylated styrene-butadiene latex, as described in U.S. Pat. No. 4,990,541 and European patent 0 427 219. There, superabsorbent polymer is introduced to a frothed, wet latex by spraying onto it the solid SAP powder prior to drying of the froth. That process uses a SAP powder of a particle size generally less than about 30 microns, and at a powder loading of approximately 25% based on the foam weight. Handling and processing of fine, superabsorbent powder particles of approximately 30 micrometers (microns) in size is difficult, especially on a large scale, partly because fine SAP powder rapidly absorbs moisture from the air and its particles agglomerate. Handling of such fine powder also increases potential processability issues. These include possible dust inhalation issues (respirable fines), and the issue of possible powder explosiveness that arises when a fine dust is handled. Adding and mixing SAP powder to a wet latex froth can also result in froth breakdown and any open-cell structure of resultant foam can be destroyed. Due to the quick absorption of water by the fine SAP particles, the SAP addition procedure does not permit a suitably controlled distribution of such powder on the wet foam surfaces. The relatively low amount of superabsorbent polymer that can be added to such a frothed latex also limits use of that system to those applications requiring only a relatively low absorbency capability.
U.S. Pat. Nos. 6,033,769 and 5,763,067 and German patent 4 418 319 relate to a layered foam composite comprising a water-soluble polymer foam layer and a SAP particles layer. The water-soluble polymer, typically a polyvinyl alcohol or carboxymethyl cellulose, is frothed and spread in a frothed sheet layer. Then the SAP particles are sprinkled through a template onto the froth and fixed by heat treatment. Such water-soluble polymer foams typically lack flexibility and need to be treated with a large amount of plasticizer to give a non-brittle foam; for example, a plasticizer amount of more than 200% based on the foam weight. Such water-soluble polymer foams can dissolve when contacted with aqueous liquid, causing a potential foam stability issue. Migration of such water-soluble polymer material in a hygiene article can create a slimy feeling for the user and is undesirable.
Both U.S. Pat. No. 5,149,335 and European patent 0 962 206 relate to an absorbent structure comprising containment means for aqueous fluid that can contain a relatively high concentration of SAP. The patents describe the use of a SAP with certain absorbency characteristics when it is desired to employ the SAP at relatively high concentrations. The U.S. patent employs specially agglomerated fines of SAP in an organic suspension medium. The containment means is made by pressing two layers of cellulose tissues, in which SAP particles are placed onto the bottom tissue layer in a patterned fashion and that layer is then pressed together, with the upper tissue layer bonding to the bottom layer with hot melt adhesive. The containment means described in both patents utilize a diaper design comprising an absorbent core of cellulose fluff pulp and SAP particles.